1. Field of the Invention
This invention relates to a magnetic rotary encoder for detecting relative movement of an object member for detection to provide a corresponding electric output signal, and more particularly to a magnetic rotary encoder wherein the output characteristic thereof is improved so as to be free from the contact force of an object member for detection.
2. Description of the Prior Art
A rotary encoder which provides an electric output signal indicative of relative movement of an object member for detection is employed in a coordinate input device of the manually operated type called mouse track ball, an image scanner which detects relative movement between an optical record medium and an optical detector to provide output picture image information, and like appliances. Means for detecting such movement as described above include the rotary switch type and the rotary volume type wherein an electric output signal is produced in accordance with a change of the position of electric contact, and the capacitance type, the optical detection type and the magnetic type wherein an electric output signal is produced without contacting with a rotary shaft.
In these days, development of a rotary encoder of the magnetic type which converts an amount of movement into a change of magnetism to provide a corresponding electric output and is advantageous in regard to power consumption and configuration has been made lively. A coordinate input device of the manually operated type (mouse) in which such a magnetic rotary encoder is employed has such a construction as described below.
In particular, a mouse in which a magnetic rotary encoder is employed normally includes a ball which is disposed for rotation in a casing with an end portion thereof projected outwardly of the casing and serves as an object member for detection. The mouse further includes a rotary shaft which is rotated in response to rotation of the ball, a magnetic scale for converting a rotational angle of the rotary shaft into an amount of change of magnetism, and a magnetic sensor for detecting a quantity of magnetism of the magnetic scale. The rotary shaft, magnetic scale and magnetic sensor are also accommodated in the casing. With such a mouse which employs a magnetic rotary encoder of the type mentioned, an end portion of the ball projected outwardly of the casing of the magnetic rotary encoder is contacted with and turned by, for example, a sheet for exclusive use in order to provide an output signal indicative of an amount of movement of a surface of the ball to a graphic display unit.
In such a conventional magnetic rotary encoder as described above, however, since the rotary shaft is disposed for rotation with the opposite ends thereof supported for rotation by a pair of bearings within the casing, the magnetic rotary encoder has a drawback that it has a comparatively great overall size and is produced at a comparatively high cost. In particular, since high durability is required for the bearings on which the opposite ends of the rotary shaft are supported for rotation, the production cost is high and the overall size of the magnetic rotary encoder is great accordingly.
Further, since a component of force of the force which is applied to a substantially central location of the rotary shaft when the ball is rotated is applied to each of the bearings at the opposite ends of the rotary shaft so that a torsional force around an axis of the rotary shaft is imparted to each of the bearings by the rotary shaft which is yielded by a force from the ball, the smoothness of rotation is deteriorated and the life of the bearings is also deteriorated. Further, if the rotary shaft is yielded by a force applied thereto from the ball when the ball is rotated, the distance between a magnetic reluctance sensor secured within the casing and a magnetic scale mounted on a circumferential face of the rotary shaft fluctuates significantly. Consequently, the detection sensitivity of the magnetic rotary encoder fluctuates and a stable detection characteristic is not obtained.
Meanwhile, a coordinate input device in which a conventional magnetic rotary encoder is used as it is has a drawback that it has a comparatively great size and the production cost is comparatively high because first and second rotary shafts are disposed within a casing with the opposite ends thereof supported for rotation by a pair of bearings.
Thus, where a conventional rotational angle detecting means which has, for example, an electric contact, is employed, the first and second rotary shafts are mounted for rotation by way of a pair of bearings on a mounting plate of the casing and transmission gears are provided at end portions of the rotary shafts such that sliding shafts of variable resistors may be rotated by way of the transmission gears. The variable resistors must necessarily be fixedly disposed at locations on the mounting plate of the casing near the end portions of the first and second rotary shafts.
In this instance, the two variable resistors must be disposed with a sufficient spatial room within the casing so that they may not obstruct free rotation of the rotary ball. Accordingly, the first and second rotary shafts have corresponding great axial lengths, and the bearings for supporting the opposite ends of the rotary shafts for rotation are required to be ball bearings which are superior in durability. Consequently, the coordinate input device is high in production cost and has a great overall size accordingly.